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United States Patent |
5,610,615
|
Chiodini
|
March 11, 1997
|
Method and device for mobile station positioning by means of a
satellite, and associated transmission method
Abstract
A positioning method for a mobile station in the radio coverage area of a
satellite entails determining the instantaneous distance between the
satellite and the mobile station. The instantaneous elevation angle .phi.
of the satellite is then calculated from the instantaneous distance, the
altitude of the satellite and the radius of the Earth. The Doppler shift
.delta. is then determined, after which the angle .theta. between the
projections onto the surface of the Earth of the track of the satellite
and a straight line segment passing through the satellite and the mobile
station is calculated from the Doppler shift .delta. and the instantaneous
elevation angle .phi.. The location of the mobile station is then
determined from the instantaneous distance, the instantaneous elevation
angle .phi. and the angle .theta..
Inventors:
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Chiodini; Alain (Boulogne, FR)
|
Assignee:
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Alcatel Mobile Communication France (Paris, FR)
|
Appl. No.:
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492886 |
Filed:
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June 20, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
342/357.17; 342/352; 342/357.16 |
Intern'l Class: |
G01S 005/02 |
Field of Search: |
342/352,357,354
455/12.1
|
References Cited
U.S. Patent Documents
4386355 | May., 1983 | Drew et al.
| |
4819053 | Apr., 1989 | Halavais | 342/353.
|
5161248 | Nov., 1992 | Bertiger et al. | 455/17.
|
5548801 | Aug., 1996 | Araki et al. | 455/13.
|
Foreign Patent Documents |
0250105A1 | Dec., 1987 | EP.
| |
Other References
Proceedings of the IEEE, vol. 60, No. 5, May 1972, New York US, pp.
564-571, Ehrlich, "The Role of Time-Frequency in Satellite Position
Determination Systems".
|
Primary Examiner: Tarcza; Thomas H.
Assistant Examiner: Pham; Dao L.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
There is claimed:
1. Positioning method for a mobile station in the radio coverage area of a
satellite, comprising the following steps:
determining the instantaneous distance between said satellite and said
mobile station;
calculating the instantaneous elevation angle .phi. of said satellite from
said instantaneous distance, the altitude of said satellite and the radius
of the Earth;
determining the Doppler shift .delta.;
calculating the angle .theta. between the projections onto the surface of
the Earth of the track of said satellite and a straight line segment
passing through said satellite and said mobile station from said Doppler
shift .delta. and said instantaneous elevation angle .phi.; and
determining the location of said mobile station from said instantaneous
distance, said instantaneous elevation angle .phi. and said angle .theta..
2. Method according to claim 1 wherein said step of calculating said angle
.theta. comprises the following steps:
calculating the value of cos(.theta.) from the Doppler shift equation:
.delta.=v.sub.s /c.f.sub.p. cos (.theta.). cos (.phi.)
where:
v.sub.s is the speed of said satellite:
c is the speed of light; and
f.sub.p is the carrier frequency of the transmitted signal; and
resolving uncertainty as to the sign of said angle .theta..
3. Method according to claim 2 further comprising the following steps:
dividing said radio coverage area into two separate regions, a first of
which corresponds to a positive value of .theta. and a second of which
corresponds to a negative value of .theta.; and
assigning a separate synchronization signal to each of said regions, and
wherein said uncertainty resolving step comprises a step of analyzing the
received synchronization signal to determine the region in which said
mobile station is located and thereby the sign of .theta..
4. Method according to claim 3 wherein the separation between said regions
is defined by the track of said satellite.
5. Method according to claim 1 wherein said step of determining said
instantaneous distance comprises a step of measuring the transit time of a
signal transmitted between said satellite and said mobile station.
6. Method according to claim 1 wherein said step of determining said
Doppler shift .delta. uses a pilot signal comprising two juxtaposed
temporally symmetrical signal elements of equal duration.
7. Method according to claim 1 wherein said satellite provides a mobile
radio service.
8. Method according to claim 1 wherein said satellite conforms to the
Globalstar system standards.
9. Mobile station positioning device utilizing the method according to
claim 1.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of positioning (i.e. determining the
geographical location of) mobile stations using radiolocation techniques.
These techniques enable a mobile station to determine its geographical
location at any time using radio links between the mobile station and one
or more reference points.
2. Description of the Prior Art
Many positioning systems exist already. They are essentially intended for
maritime and aeronautical applications and use a specific transmission
infrastructure such as a network of terrestrial transmitters (as in the
OMEGA or LORAN C systems, for example) or a constellation of satellites
(as in the TRANSIT system, for example).
The existing Global Positioning System (GPS) can be used in any type of
mobile station. It uses a constellation of 24 satellites deployed in such
a manner that a mobile station can receive signals from three separate
satellites at any time. In the GPS system three satellites are required
for positioning to be possible.
A major drawback of these prior art dedicated systems is that they require
a very high investment in terms of transmission network installation and
maintenance.
These infrastructure costs also impact on the positioning devices.
Furthermore, these devices must include powerful and accurate computation
means since they have to achieve an accuracy in the order of a few tens of
meters.
This degree of accuracy is of benefit in some applications. There is,
nevertheless, an entirely separate requirement for coarser positioning
(for example, at the level of one cell in a cellular mobile radio network)
at much lower cost. At present there is no system meeting this need.
One object of the invention is to meet this need and to alleviate the
various drawbacks of the prior art.
To be more precise, one object of the invention is to provide a positioning
method using a simplified satellite infrastructure. In other words, one
object of the invention is to provide a positioning method requiring
reception of signals from only one satellite (rather than from three
satellites, as in the prior art systems).
Another object of the invention is to provide a method of this kind such
that the design and manufacture of the positioning devices combine
simplicity with low cost. In particular, one object of the invention is to
provide a positioning method which can be implemented easily in mobile
telephones.
A further object of the invention is to provide a positioning method which
does not require a specific infrastructure but which can use an existing
infrastructure such as the Globalstar network, for example.
SUMMARY OF THE INVENTION
These objects, and others that emerge below, are achieved in accordance
with the invention by a positioning method for a mobile station in the
radio coverage area of a satellite, comprising the following steps:
determining the instantaneous distance between said satellite and said
mobile station;
calculating the instantaneous elevation angle .phi. of said satellite from
said instantaneous distance, the altitude of said satellite and the radius
of the Earth;
determining the Doppler shift .delta.;
calculating the angle .theta. between the projections onto the surface of
the Earth of the track of said satellite and a straight line segment
passing through said satellite and said mobile station from said Doppler
shift .delta. and said instantaneous elevation angle .phi.; and
determining the location of said mobile station from said instantaneous
distance, said instantaneous elevation angle .phi. and said angle .theta..
Thus, in accordance with the invention, mobile station positioning is made
possible using a single satellite by determining and using the Doppler
shift.
Said step of calculating said angle .theta. advantageously comprises the
following steps:
calculating the value of cos(.theta.) from the Doppler shift equation:
.delta.=v.sub.s /c.f.sub.p. cos (.theta.). cos (.phi.)
where:
v.sub.s is the speed of said satellite:
c is the speed of light; and
f.sub.p is the carrier frequency of the transmitted signal; and
resolving uncertainty as to the sign of said angle .theta..
The sign of .theta. varies according to whether the mobile station is on
the right or on the left of the projection of the satellite track in the
coverage area.
Resolving uncertainty as to the sign of .theta. is preferably made possible
by adding the following steps to the method:
dividing said radio coverage area into two separate regions, a first of
which corresponds to a positive value of .theta. and a second of which
corresponds to a negative value of .theta.; and
assigning a separate synchronization signal to each of said regions.
In this case, said uncertainty resolving step comprises a step of analyzing
the received synchronization signal to determine the region in which said
mobile station is located and thereby the sign of .theta..
The separation between said regions is preferably defined by the track of
said satellite.
Said step of determining said instantaneous distance advantageously
comprises a step of measuring the transit time of a signal transmitted
between said satellite and said mobile station.
Said step of determining said Doppler shift .delta. preferably uses a pilot
signal comprising two juxtaposed temporally symmetrical signal elements of
equal duration.
This method can be used in the context of a mobile radio service such as
the Globalstar system.
The invention also consists in mobile station positioning devices utilizing
the method as defined hereinabove and a method of transmitting digital
signals from a satellite, comprising the following steps:
dividing said radio coverage area into two separate regions, one on each
side of the track of said satellite; and
assigning a different synchronization signal to each of said regions.
Other features and advantages of the invention will emerge from a reading
of the following description of a preferred embodiment of the invention
given by way of non-limiting illustrative example only and from the
appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the radio coverage area of a satellite.
FIG. 2 shows three parameters d, .phi. and .theta. for positioning a mobile
station relative to a satellite.
FIG. 3 is a flowchart of the positioning method of the invention.
FIG. 4 shows the principle utilized by the invention to resolve uncertainty
as to the sign of the instantaneous elevation angle.
FIG. 5 shows a specific instance of the FIG. 4 principle corresponding to
the Globalstar mobile radio system.
FIGS. 6 through 8 show a method of determining the Doppler shift:
FIG. 6 shows the structure of a prior art pilot signal for a CDMA mobile
radio system;
FIG. 7 shows the structure of a pilot signal for using this method of
determining the Doppler shift, again for a CDMA mobile radio system;
FIG. 8 is a functional block diagram of a frequency and time
synchronization device producing the FIG. 7 signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Mobile station positioning is one service proposed for future satellite
mobile radio systems. On a more general level, satellite-based positioning
is a technique with considerable growth potential. The invention proposes
a new positioning method using signals received from only one satellite.
The embodiment of the invention described below is intended for use in the
Globalstar system, which is a CDMA cellular mobile radio system offering
global coverage. It comprises three segments:
the space segment, comprising 48 satellites in low Earth orbit (at 1 414 km
altitude), eight back-up satellites and two constellation control centers,
portable terminals installed on mobile stations or fixed stations, which
are either mono-mode terminals (i.e. capable of interworking only with
Globalstar) or bi-mode terminals (i.e. capable of interworking with
Globalstar and a terrestrial cellular system such as the GSM or DCS 1800
system),
connection stations which set up links to public switched networks and
manage mobility, the latter function entailing the updating of databases
indicating the present location of a mobile and the attributes of the
services to which it has subscribed. This capacity enables integration
into mobile networks. This enables bi-mode subscribers to retain their
existing mobile phone number and to be called either from within the
terrestrial network or from within the satellite network without having to
do anything about it.
The basic Globalstar service is a telephone service. There is also
provision for transmission of data. Globalstar will in the future offer a
new service, namely extension of roaming worldwide. However, it is clear
that this extension will be restricted to the service areas (i.e. outside
dense conurbations in which coverage is in theory provided by a cellular
network). Outside these service areas, a call request signal will be
supplied by a one-way radiopaging service.
Because of its transparency and its integration into public networks, the
Globalstar system will also be possible to offer the same services as the
cellular networks.
The invention adds a further function to this system, namely mobile station
positioning.
FIG. 1 shows the radio coverage area of the satellite. To a first
approximation this is a disk 11 comprising a top half-disk 12 and a bottom
half-disk 13.
The top half-disk 12 defines an area within which the signal received by
the terminal from the satellite is affected by a positive Doppler shift
(the satellite is moving towards the terminal).
The bottom half-disk 13 defines an area in which the signal received by the
terminal from the satellite is affected by a negative Doppler shift (the
satellite is moving away from the terminal).
The two half-disks 12 and 13 are defined by the null Doppler shift diameter
14 which is perpendicular to the track 15 of the satellite.
FIG. 2 shows the parameters needed for mobile station positioning in the
known manner. Note that the invention is not specifically concerned with
the positioning method as such, but with how the various parameters
required are determined.
The instantaneous position of the mobile station 21 relative to the
satellite 22 passing overhead is entirely defined by the following three
parameters:
the instantaneous distance d between the satellite 22 and the mobile
station 21;
the instantaneous elevation .phi. of the satellite 22 relative to the
terminal 21;
the angle .theta. between the projections onto the surface of the Earth of
the track 23 of the satellite and the satellite-terminal direction 24.
In accordance with the invention, these parameters are determined by the
method shown in FIG. 3.
This method comprises two types of processing. Firstly, initialization
processing (31) carried out once and for all on board the satellite to
resolve the uncertainty as to the sign of .theta., as described below, and
secondly positioning processing (32) which can be carried out at any time
and by any mobile station equipped with the necessary processing means.
Mobile station positioning processing (32) in accordance with the invention
thus entails determining the position of a mobile station from signals
transmitted by a single satellite.
To this end, the first step 33 of the method determines the distance d
between the satellite and the mobile station.
This distance can be determined in a conventional way, for example by
measuring the transit time of a signal transmitted between the satellite
and the terminal.
The next step (34) calculates the instantaneous elevation .phi. of the
satellite relative to the terminal, from the value d, the radius R of the
Earth and the altitude h of the satellite. The data R and h is known to
the mobile terminal if the latter is responsible for all positioning
processing. More generally, in this case, the ephemerides of the satellite
used are known to the terminal.
To restrict the complexity of the terminal, most of the processing entailed
in mobile station positioning can be carried out by the corresponding
Earth station.
The Doppler shift .delta. is determined by the mobile station at the time
of initial synchronization and at regular intervals thereafter. A method
of determining the Doppler shift is described below.
In this method, the satellite transmits a pilot signal comprising
temporally symmetrical signal elements, for example at least one
pseudo-random first digital synchronization sequence x(0) through x(N-1)
and, periodically, at least one second digital sequence x(N-1) through
x(0) temporally symmetrical with (i.e. constituting a mirror image of)
said first sequence.
Appropriate analysis of this signal structure recovers the timing reference
of the base station and measures the Doppler shift.
The Doppler shift .delta. is defined by the equation:
.delta.=v.sub.s /c.fp. cos (.theta.). cos (.phi.) (1)
where:
v.sub.s is the speed of the satellite;
c is the speed of light;
f.sub.p is the carrier frequency.
The values of v.sub.s, c and f.sub.p are known to the terminal. It is
therefore possible (in step 36) to calculate cos (.theta.) from equation
(1) and therefore to calculate the absolute value of .theta..
There is therefore some uncertainty as to the sign of .theta.. In
accordance with an important feature of the invention, this uncertainty
can be resolved by using two specific pilot signals A and B and dividing
the radio coverage area into two areas relative to the satellite track as
shown in FIG. 4:
the synchronization signal A is transmitted only into the half-disk 42 on
the righthand side of the track 41; and
the synchronization signal B is transmitted only into the half-disk 43 on
the lefthand side of the track 41.
This corresponds to the initialization step (31), during which the coverage
area is divided into two separate areas (39), to which different
synchronization signals are assigned (310), the synchronization signals
being known to the terminals, of course. In the specific case of the
Globalstar system, the radio coverage area resembles a disk made up of 19
beams 51.sub.1 through 51.sub.19, in the manner shown in FIG. 5.
The pilot signals A and B are distributed in a similar manner, on either
side of the track 52 of the satellite.
Accordingly, the positioning process (32) includes a step (37) of resolving
the uncertainty as to the sign of .theta. by analyzing the frequency
characteristics of the pilot signal received, enabling the terminal to
determine in which of the half-disks 42 and 43 it is located.
The position of the mobile station can be determined (step 38) knowing the
three values (.theta., .phi., .delta.), as previously mentioned.
This latter step is advantageously implemented by the Earth station. In
this case, the role of the terminal is restricted to:
enabling measurement of the satellite/mobile station transit time by the
Earth station;
measuring the Doppler shift (sign and absolute value);
determining which synchronization signal has been detected (A or B); and
transmitting this information to the Earth station.
The Earth station can calculate the position of the mobile station from the
known transit time, Doppler shift, detected synchronization signal,
satellite ephemerides and radius of the Earth. The Earth station can
transmit this position to the mobile station, if necessary.
A method of determining the Doppler shift is described next with reference
to FIGS. 6 through 8.
FIG. 6 shows the structure of a pilot signal as used in a prior art CDMA
system such as the Globalstar system. The pilot signal is formed by the
continuous repetition of the same pseudo-random sequence (PN) 11'
comprising a series of N.sub.c bits 12'.sub.0 to 12'N.sub.c-1.
The sequence PN can be a sequence of 32 767 (10.sup.15 -1) bits supplied by
an appropriate generator polynomial, for example. The receivers know the
sequence and can therefore synchronize to the received signal. This refers
to time synchronization. Frequency synchronization is effected
independently, and no reliable information is available as to the Doppler
shift.
A new synchronization signal shown in FIG. 7 enables frequency and time
synchronization of the receivers.
This signal still comprises a first (direct) sequence PN(n) 21' transmitted
regularly. However, this first sequence 21' is replaced at regular
intervals by a second (inverse) sequence PN(N.sub.c -1-n) 22' obtained
from the first sequence PN by temporal symmetry.
The symmetrical second sequence can be obtained at the binary level
(reversed order of binary reading) or directly at the modulated signal
level.
It is inserted regularly into the pilot signal at a rate which depends on
the needs of the system (in terms of the number of false detections and
acceptable Doppler shift, for example). In the FIG. 7 embodiment the
sequences transmitted are alternately the direct sequence 21' and the
inverse sequence 22'.
FIG. 8 is a diagrammatic representation of a synchronization device using
the FIG. 7 signal.
The received pilot signal 31' is sampled by an analog-digital converter 32'
which supplies samples x(i) with a sampling period T.sub.e.
These samples are fed into a first shift register 33' with N cells (N is
the length of the sequences PN). The sampling period is chosen so that
T.sub.PN =N.T.sub.e corresponds to the duration of the sequence PN.
The output of the register 33' is looped (34') to the input of a second
shift register (35') so having N cells. Thus, at a given time:
the register 33' contains the samples x(0) through x(N-1); and
the register 35' contains the samples x(N) through x(2N-1).
The device further comprises multiplier means 36'.sub.0 through 36'.sub.N-1
each of which multiplies the contents of two cells of the registers 33'
and 35' to generate N coefficients c(i) 37' such that:
c(i)=x(i).x(2N-1-i)
with i varying from 0 through N-1.
These coefficients can be written as follows:
##EQU1##
When, at a given time, the shift register 33' contains a direct sequence
exactly and the shift register 35' contains an inverse sequence exactly
(this is referred to as "coincidence"), the following equality applies:
.phi.(nT.sub.e)=.phi.[(2N-1-n)T.sub.e ]
The multiplier output signal is then:
c(nT.sub.e)=1/2. cos (2 .pi..delta. (2N-1-n)T.sub.e +2.phi.(nT.sub.e))+1/2.
cos (4 .pi..delta. nT.sub. -2 .pi..delta. (2N-1-n)T.sub.e)
It is a simple matter to verify that:
the first term is a spread spectrum signal; and
the second term is a pure sinusoid, a function of 2.delta..
When there is no coincidence, on the other hand, only a spread spectrum
signal is obtained.
The sequence of coefficients from the multiplier is passed to spectrum
analysis means 38'. Spectrum analysis at intervals T.sub.e enables, using
conventional methods:
acquisition of the timing reference of the base station, as soon as
coincidence occurs; and
measurement of the Doppler shift, by measuring the frequency of the
sinusoid which is equal to 2.delta..
Note that it is not mandatory to insert the inverse sequence into the pilot
signal. In a different embodiment of the invention, the inverse sequence
can be transmitted on another frequency. In this case the pilot signal
comprising only the direct sequence is transmitted continuously and the
inverse sequence is transmitted at least periodically.
In this case, the device of FIG. 8 must naturally be adapted accordingly,
the two shift registers being no longer in cascade but fed independently
with each sequence.
More generally, different constructions of the device are feasible provided
that the coefficients c(i) are calculated and then analyzed.
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